The present invention relates to a control method for an electric power steering apparatus for a vehicle.
In general, the convergence of a vehicle obtained by a steering operation in a straight direction or a running direction is degraded along with an increase in vehicle speed. One cause is that a self-aligning torque (force of returning a steering wheel) is increased as a vehicle speed is increased, and a steering wheel is strongly returned after a steering operation, thereby causing an overshoot phenomenon (in which a steering angle once returning to the center goes beyond the center toward the opposite side). If a driver releases his or her hands from the steering wheel, the overshoot phenomenon repetitively occurs, and the vehicle travels in a zigzag manner.
In an electric power steering apparatus, since the inertia of an electric power steering unit constituted by a motor and reduction gears is large, this tendency of overshoot conspicuously appears.
If a uniform damper is provided in order to prevent this phenomenon, a steering wheel cannot be lightly turned in a low-speed condition, and a sufficient convergence cannot be obtained in a high-speed condition.
The present applicants, therefore, have previously proposed Japanese Patent Laid-Open No. 1-178081 (corresponding to U.S. Ser. No. 07/290,697: and its continuing application Ser. No. 07/798,026) in order to obtain an electric power steering apparatus having good convergence in both high and low-speed conditions. That is, in a control method for this electric power steering apparatus for a vehicle (to be referred to as a prior application hereinafter), a damper (viscous element) for braking a motor is arranged for control, so that damping (viscosity) is changed along with an increase in vehicle speed.
FIG. 5 shows blocks for explaining the principle of the prior application. Referring to FIG. 5, reference numeral 1 denotes a steering wheel; 2, a torque sensor for detecting a torque based on an operation of the steering wheel 1; 3, a control unit for controlling an electric power steering unit 4 on the basis of an output from the torque sensor 2; 3a, a target value generator for generating a target value Tt of a steering force or a motor output; and 3b, a primary delay system setting means for setting a primary delay system having a variable damper value F1 corresponding to a vehicle speed. The electric power steering unit 4 includes a pinion 4b engaged with a rack 4a which is coupled to wheels, a motor 4c, and reduction gears 4d arranged between the motor 4c and the pinion 4b, as is known to those skilled in the art.
In this arrangement, the electric power steering unit 4 is controlled using a value Z0 obtained by filtering a difference between the target value Tt and an actual value Td through the primary delay system.
In the prior application, the output value Z0 from the primary delay system is multiplied by the damper value F1, and this product Z0.F1 is fed back to a difference P0 between the target value Tt and the actual value Td. In addition, as shown in FIG. 6, in a low-speed range from vehicle speed V0 to V1 (V0&lt;V1), the damper value F1 is set positive and decreased as the vehicle speed is increased.
As a result, in this prior application, a response time is improved in a low-speed condition (V0 to V1) in which steering wheel return control is required.
The output value Z0 from the primary delay system, however, is merely a target value of control and therefore does not always correspond to an actual value. For this reason, when the product of Z0 and the damper value F1 is fed back to the difference P0 between the target value Tt and the actual value Td, viscosity control for the electric power steering unit 4 sometimes becomes incorrect. Furthermore, an input amount to the primary delay system is (Tt-Td)-Z0.multidot.F1, i.e., Z0-F1 is subtracted as a positive value. The result is an unsatisfactory response time of steering wheel return control in a low-speed condition.